Due to their known advantages, such as high efficiency in terms of lumens per watt, small form factor and durability, LEDs are used as light sources in high performance lighting fixtures. LEDs are increasing preferred light sources in difficult-to-replace lighting fixtures, such as street lights, traffic signal lights and in fixture that require high reliability, such as automotive lights, for instance for safety reasons.
Similar to many other light sources, the light output from an LED decays over time, ultimately leading to LED failure. In order to avoid complete failure, LEDs are typically replaced according to a fixed schedule. However, since the replacement schedules generally try to completely avoid pre-replacement failure, and there is a significant spread in the time at which an LED may be expected to fail, many LEDs are replaced considerable before a likely failure, which is clearly wasteful; alternatively, if the replacement schedule is extended in order to reduce such waste, some LEDs are likely to fail before being replaced, which is generally inconvenient and could be dangerous.
In order to predict, and thereby where appropriate prevent, the failure of LEDs, it is known to monitor or measure the light output by means of external optical sensors such as photodiodes. Whilst this method is generally robust, it requires additional components, circuitry and wiring, and is thus undesirable. Further, in non-ideal lighting environments, such as where there may be interference from other LEDs or extraneous light sources, the method may be inaccurate.
There is thus an ongoing requirement to provide other methods of predicting the failure of LEDs, and characterising their performance in general.